A simple technique is developed for etching the overcladding layer of aperﬂuorinated graded-index polymer optical ﬁbre by the combined useof chloroform and water. The boundary between the two liquids canmitigate the evaporation of chloroform, leading to a much lower loss
than that of the conventional technique which does not employ
water. The effectiveness of this method is veriﬁed by Brillouinsignal measurement and infrared thermometry.

Introduction: Owing to their measurability of strain and temperature
distribution along the ﬁbres, ﬁbre-optic sensors based on Brillouin scattering have been widely studied as a promising technique for healthmonitoring of civil structures, such as buildings, dams and aircraft
wings [1–4]. Their sensor heads have been mainly composed of glass
optical ﬁbres, which are so fragile that they cannot withstand strainsof over several percentages. To extend the measurable/endurable
strain, we have been focusing on the use of Brillouin scattering inpolymer optical ﬁbres (POFs), which have extremely high ﬂexibility
and can withstand strains of > 50% [5]. After succeeding in observing
Brillouin scattering in perﬂuorinated graded-index (PFGI-) POFs at
1.55 μm [6], we have shown that it can be potentially applied to highprecision temperature sensing [7] as well as large-strain sensing [8].
The biggest problem that makes such practical applications difﬁcult is,however, their extremely weak Brillouin signal because of their large
core diameters and multimode nature.
One method for enhancing the Brillouin signal is to taper thePFGI-POFs. Heat-assisted tapering is one of the most major methods
for tapering glass optical ﬁbres [9], which has been shown to be also
applicable to poly(methyl methacrylate)-based (PMMA-) POFs [10](observing Brillouin scattering in PMMA-POFs is extremely difﬁcult[11]). However, this method cannot be directly used to taper commercialised PFGI-POFs, because the glass-transition temperatures of thecore/cladding layer (amorphous perﬂuorinated polymer) and the overcladding layer (polycarbonate) are different ( < 108 and 144 °C, respectively), resulting in the destruction of the wave-guiding structure of thecore. The pressing need for the stable fabrication of tapered
PFGI-POFs is to develop a technique for removing their overcladdinglayers. Chloroform has been conventionally used [12], but with ahigher loss, which needs to be mitigated for Brillouin applications.
In this Letter, a simple technique for etching the overcladding layer ofa PFGI-POF is demonstrated by using chloroform and water. A much
lower loss is expected because the boundary between the two liquids
suppresses the evaporation of chloroform. We experimentally show
that, using this method, the Brillouin signal in a PFGI-POF withoutthe overcladding layer can be clearly observed. We also prove the effectiveness of this method by infrared thermometry.
Experimental setup: We employed two 1.05 m-long PFGI-POFs as theﬁbres under test, which had a numerical aperture of 0.185, a core diameter of 50 μm, a cladding diameter of 100 μm, an overcladding diameterof 750 μm, a core refractive index of ∼ 1.35 and a propagation loss of∼ 250 dB/km at 1.55 μm. The core/cladding layers and the overcladding
layer were, respectively, composed of amorphous perﬂuorinatedpolymer (polymer ﬂuorobutenylvinyl ether; commercially known asCYTOP) and polycarbonate [12].
Fig. 1a shows the schematic setup for etching the overcladding layerof the ﬁbre under test (FUT) using chloroform only. A 0.9 m-longportion of the FUT was perpendicularly dipped into 500 ml of chloroform (Wako Pure Chemical Industries Ltd) in a container for
20 minutes (sufﬁciently long to remove its overcladding layer). Note
that, during this process, the overcladding layer near the surface of thechloroform was partially damaged by the evaporated chloroform, resulting in a high optical loss. Then, the FUT was taken out, ﬁxed perpendicularly and dried in air. The ﬁbre end of the etched portion wasangled-cut to suppress the possible Fresnel reﬂection; the ﬁbre end of
the unetched portion ( ∼ 0.15 m) was butt-coupled to a silica singlemode ﬁbre via an SC connector, from which 1.55 μm light was injected.
In contrast, Fig. 1b shows the setup for etching the overcladding layer byusing both chloroform and water. We added 500 ml of water (upper:PFGI-POFwithovercladding

airevaporated
chloroform

PFGI-POF
withovercladding

airwaterdamagedregionchloroform

chloroform
PFGI-POF
without
overcladding

PFGI-POF
without
overcladding

containercontainerabFig. 1 Schematic setups for etching overcladding layers of PFGI-POFs using
chloroform only, and chloroform and watera Chloroform onlyb Chloroform and water

Experimental results: Fig. 2a shows the BGSs measured (i) before
etching the overcladding layer, (ii) after etching it by using chloroform
and water and (iii) after etching it by using chloroform only. The pumppower was ﬁxed at 28.5 dBm. The Brillouin frequency shift was∼ 2.8 GHz, which is in good agreement with the previous results [6].
Although it is clear that the Stokes power was reduced by etching theovercladding layer, the Stokes power of (ii) was approximately 1.5 dB
higher than that of (iii). The Stokes power of (iii) seems to be attributed
to the ∼ 0.15 m-long unetched portion, because the BGS of (iii) was
almost the same as that measured using a 0.15 m-long unetched
PFGI-POF sample shown, see Fig. 2b. These results indicate that, in(ii), the Brillouin signal in the PFGI-POF with its overcladding layer
etched was successfully observed for the ﬁrst time.
–55

–55
–56

pump:
28.5 dBm(i)–57

Stokes power, dBm

N. Hayashi, Y. Mizuno and K. Nakamura

speciﬁc gravity SG = 1.0) to 500 ml of chloroform (lower: SG = 1.46)
in a container, and the same experimental procedures were followed.
Next, the Brillouin gain spectra (BGSs) of the ﬁbres under test [13]with the overcladding layers etched by the two different methods werecompared. The experimental setup for measuring the BGSs, based on
self-heterodyne detection, was basically the same as that previously
reported [6]. The polarisation state was optimised for each measurementwith the polarisation controllers so that the Stokes power might bemaximal. The temperature was kept at 25 °C for all the measurements.

Stokes power, dBm

Improved technique for etchingovercladding layer of perfluorinated polymer
optical fibre by chloroform and water

(ii)

–58
(iii)

–59
–60
2.6

2.7
2.8
2.9
frequency, GHz3.0–56

pump:
28.5 dBm

–57
–58
–59
–60
2.6

a2.9
2.7
2.8
frequency, GHz

3.0bFig. 2 BGS measurementsa BGSs measured (i) before etching, (ii) after etching using chloroform and waterand (iii) after etching using chloroform onlyb BGS measured in 0.15 m-long unetched PFGI-POF

Discussion: Fig. 3a is a photograph of the PFGI-POFs, along which
28.5 dBm light beams were propagating. The overcladding layer ofthe left ﬁbre was etched using chloroform only, whereas that of theright ﬁbre was etched using both chloroform and water. Scarcely any
difference was found between their appearances. Then, to verify thelocation of the induced loss, we obtained the infrared thermal image[14] of the same PFGI-POFs, as shown in Fig. 3b. Almost no local
increase in temperature was detected in the right ﬁbre, while a clear
bright spot was observed in the left ﬁbre. Thus, we found that a considerable loss is induced at the etched/unetched boundary of a PFGI-POF
when only chloroform is used to etch its overcladding layer, and that
this loss can be greatly mitigated by the combined use of chloroform
and water, which suppresses the evaporation of chloroform.

ELECTRONICS LETTERS 5th December 2013 Vol. 49 No. 25 pp. 1630–1632

References
only chloroform

39.0 °Cheat notdetectedchloroform
and water
5 mm

26 °C

a45 °CbFig. 3 Photograph and infrared thermal image, of PFGI-POFs with overcladding layers etched using chloroform only (left), and chloroform andwater (right)
a Photographb Infrared thermal image

Conclusion: We have demonstrated a simple technique for etching theovercladding layer of a PFGI-POF by using chloroform and water. Amuch lower loss can be obtained than that in the conventional technique
by employing chloroform only, because the boundary between the twoliquids suppresses the evaporation of chloroform. On using this method,a Brillouin signal in the PFGI-POF with the overcladding layer etched
was clearly observed for the ﬁrst time. This method was veriﬁed to beeffective also by infrared thermometry. We believe that this result will
open the way for the development of new Brillouin devices andsystems based on tapered PFGI-POFs.
Acknowledgments: This work was partially supported by a grant-in-aidfor Young Scientists (A) (no. 25709032) from the Japan Society for thePromotion of Science (JSPS) and by research grants from the General
Sekiyu Foundation, the Iwatani Naoji Foundation and the SCAT
Foundation. N. Hayashi acknowledges a grant-in-aid for JSPS Fellows(no. 25007652).
© The Institution of Engineering and Technology 201315 October 2013
doi: 10.1049/el.2013.3267
One or more of the Figures in this Letter are available in colour online.
N. Hayashi, Y. Mizuno and K. Nakamura (Precision and Intelligence
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho,Midori-ku, Yokohama 226-8503, Japan)

1 Horiguchi, T., and Tateda, M.: ‘BOTDA – nondestructive measurementof single-mode optical ﬁber attenuation characteristics using Brillouininteraction: theory’, J. Lightwave Technol., 1989, 7, (8), pp. 1170–1176
2 Kurashima, T., Horiguchi, T., Izumita, H., Furukawa, S., andKoyamada, Y.: ‘Brillouin optical-ﬁber time-domain reﬂectometry’,
IEICE Trans. Commun., 1993, E76-B, (4), pp. 382–390
3 Garus, D., Krebber, K., and Schliep, F.: ‘Distributed sensing techniquebased on Brillouin optical-ﬁber frequency-domain analysis’, Opt. Lett.,
1996, 21, (17), pp. 1402–1404
4 Hotate, K., and Hasegawa, T.: ‘Measurement of Brillouin gain spectrum
distribution along an optical ﬁber using a correlation-based technique –proposal, experiment and simulation’, IEICE Trans. Electron., 2000,
E83-C, (3), pp. 405–412
5 Kuzyk, M.G.: ‘Polymer ﬁber optics: Materials, physics, and applications’ (CRC Press, Boca Raton, 2006)
6 Mizuno, Y., and Nakamura, K.: ‘Experimental study of Brillouin scattering in perﬂuorinated polymer optical ﬁber at telecommunication
wavelength’, Appl. Phys. Lett., 2010, 97, (2), p. 021103
7 Mizuno, Y., and Nakamura, K.: ‘Potential of Brillouin scattering inpolymer optical ﬁber for strain-insensitive high-accuracy temperaturesensing’, Opt. Lett., 2010, 35, (23), pp. 3985–3987
8 Hayashi, N., Mizuno, Y., and Nakamura, K.: ‘Brillouin gain spectrum
dependence on large strain in perﬂuorinated graded-index polymer
optical ﬁber’, Opt. Express, 2012, 20, (19), pp. 21101–21106
9 Birks, T.A., and Li, Y.W.: ‘The shape of ﬁber tapers’, J. Lightwave
Technol., 1992, 10, (4), pp. 432–438
10 Jeong, Y., Bae, S., and Oh, K.: ‘All ﬁber N × N fused tapered plastic
optical ﬁber (POF) power splitters for photodynamic therapy applications’, Curr. Appl. Phys., 2009, 9, (4), pp. e273–e275
11 Hayashi, N., Mizuno, Y., Koyama, D., and Nakamura, K.:‘Measurement of acoustic velocity in poly(methyl methacrylate)-based
polymer optical ﬁber for Brillouin frequency shift estimation’, Appl.
Phys. Express, 2011, 4, (10), p. 102501
12 Gravina, R., Testa, G., and Bernini, R.: ‘Perﬂuorinated plastic opticalﬁber tapers for evanescent wave sensing’, Sensors, 2009, 9, (12),pp. 10423–10433
13 Agrawal, G.P.: ‘Nonlinear ﬁber optics’ (Academic Press, California,1995)
14 Hayashi, N., Mizuno, Y., and Nakamura, K.: ‘Fast ﬂaw detection inpolymer optical ﬁbers with infrared thermometer’, Appl. Phys.
Express, 2013, 6, (7), p. 076601

E-mail: hayashi@sonic.pi.titech.ac.jp

ELECTRONICS LETTERS 5th December 2013 Vol. 49 No. 25 pp. 1630–1632